Self-propelled trolley vehicles of conveyor systems receive power and command signals off rails along the rack. Since power attenuates as the rail length increases due to small resistances, the rails are isolated into sections with signals supplied to each section. Therefore, vehicles which move along the sectioned track include pads contacting the rails for supply electrical signals or power to the vehicle. A pad always need to be in contact with the powered section of the rail in order to continuously supply the signals. When only a single contact pad is used, the vehicle will not receive electrical signals when over a buffer isolating the sections of rail. There are systems which use a plurality of contacting pads, wherein one pad is allowed to conduct power at a time.
One such system discloses an electric car having a pair of contact shoes which straddle the ends of two or more rail sections. Such a system is disclosed in U.S. Pat. No. 759,453 granted May 10, 1904 issued to Mahoney. These rail sections, however, are not isolated from each other. The middle rail is a closed circuit and when the vehicle passes over a section brake, it lifts the plate via magnets which brake the circuit. The circuit is completed by passing the currents through the vehicle which allows the electricity to power the vehicle motor. This prior system does not disclose any prevention of feedback through the second contacts nor with the possibility of the vehicle losing power when one or more contacts rests on a buffer.
Another such system is disclosed in U.S. Pat. No. 3,083,296 issued Nov. 21, 1960 in the name of R. A. Bradley. This system includes a train having at least two carriages. The rails are divided and alternate power and non-power sections. Each carriage has a relay and connects such that when the first carriage is open the powered rail, it conducts the power while the second carriage relay is open and non conducting, and vice verse when the second carriage is over the powered rail. Only one relay is closed at a time.
Therefore, there remains a problem when two or more contact pads are in conduction with the rail which is continuously powered by the power source and separated in section by buffers such that when any contact pad is conducting power from the rail there will not be feedback through the other contact pad and will only power the vehicle.